The invention relates to pressure controlled ink jet printheads and in particular to improved printhead assemblies and to improved methods for assembling ink jet printheads.
Ink jet technology continues to be improved in order to increase printing speed and print quality or resolution. One means for improving print speed and quality is to increase the number of nozzle holes in an ink jet printhead and to decrease the diameter of the nozzle holes. However, improvements in print speed and quality often result in operational problems not experienced with lower quality slower speed printers.
In an ink jet printer, ink is provided to the printhead from an ink cartridge or supply tank. The ink flows from the tank through a connecting conduit from the ink cartridge through an ink via in a semiconductor chip or around the edges of a semiconductor chip and into ink flow channels and an ink chamber. The ink chamber is situated in axial alignment with a corresponding nozzle hole and a heater resistor defined on the surface of the semiconductor chip. As electrical impulse energy is applied to an ink ejector adjacent an ink chamber to cause ink adjacent the ejector in the chamber to be forced through a nozzle hole onto a print medium. By selective activation of a plurality of ink ejectors on a printhead, a pattern of ink dots are applied to the print medium to form an image.
A critical aspect of the printing process is the controlled supply of ink to the ink ejectors from the ink supply. If the pressure of the ink supply is too high, ink may run out freely from the nozzle holes onto the print medium before the ink ejector is activated. If the pressure of the ink supply is too low, ink channels and chambers in the printhead will not refill fast enough. If the ink chambers and channels are not refilled fast enough there will be missing ink dots or the print speed must be lowered to allow time for ink to refill the ink chambers. Furthermore, as ink is used from the ink supply, the pressure of the ink supply may decrease to a point which inhibits flow of the remaining ink to the ink ejectors. Accordingly, as the number of nozzles holes on a printhead increases and the diameter of the holes decreases, maintaining a predetermined ink supply pressure in the ink supply system becomes more critical.
There are two primary methods for maintaining ink supply flow to printheads. The first method includes the use of a porous capillary member such as foam which is saturated with ink and provides a controlled flow of ink to the printheads. The second method includes the use of a diaphragm or bellows to provide pressure or back pressure on the ink in the cartridge. Use of a diaphragm or bellows enables an ink supply reservoir to be filled with liquid ink as opposed to the use of foam saturated with ink. Accordingly, the ink jet printhead assembly may be made smaller for the same volume of ink ejected onto the print media during the life of the printhead. One disadvantage of smaller ink jet printhead assemblies which use a diaphragm or bellows for pressure control is that manufacturing techniques become more complicated because of a greater number of different materials required for use in the printhead assembly.
There is a need therefore for an improved pressure controlled ink supply assembly and improved methods for assembling pressure controlled ink jet printheads.
With regard to the foregoing and other objects and advantages, the invention provides an ink jet printhead assembly for a pressure controlled ink jet printhead. The assembly includes an ink reservoir having an open top cavity defined by sidewalls, a bottom wall and a peripheral edge. The ink reservoir is made of a first material having a first melting point. A pressure control structure having a first surface, a second surface opposite the first surface, a side surface, and an aperture extending therethrough from the first surface to the second surface is also provided. The pressure control structure is made of a polymeric material having a second melting point lower than the first melting point. A sealing structure is provided for forming a liquid tight seal between the sidewalls of the ink reservoir and the side surface of the pressure control structure. A pressure regulating film is attached to the first surface of the pressure control structure closing the aperture therein. A cover is attached to the ink reservoir to protect the pressure regulating film from damage.
In another embodiment, the invention provides method for assembling a pressure controlled ink jet printhead assembly. The method includes providing an ink reservoir having an open top cavity defined by sidewalls, a bottom wall and a peripheral edge. The ink reservoir is made of a first material having a first melting point. A pressure control structure having a first surface, a second surface opposite the first surface, a side surface, and an aperture extending therethrough from the first surface to the second surface is also included. The pressure control structure is made of a polymeric material having a second melting point lower than the first melting point. A sealing structure is applied to the side walls of the pressure control structure for forming a liquid tight seal between the sidewalls of the ink reservoir and the side surface of the pressure control structure. The pressure control structure is inserted in the open top cavity of the ink reservoir. A pressure regulating film is attached to the first surface of the pressure control structure thereby closing the aperture therein. A cover is attached to the ink reservoir to protect the pressure regulating film from damage.
The invention provides a number of advantages over conventional ink jet printhead assemblies. For one, components of the assembly may be sub assembled and combined using only mechanical means. Also, there is no need for use of a two step injection molding process to provide inner and outer frames made of different materials. Another advantage of the invention is that the components may be assembled with a high degree of assurance of substantially no ink leakage from the assembled components. Also, a pressure control film may be made of a single material rather than from a laminate construction of two or more different materials thereby improving the pressure control response of the assembly.